In general, a hot-rolling step includes the substeps of heating a slab-like metal material produced by continuous casting, ingot making, or blooming to several hundreds to a thousand and several hundreds of degrees centigrade in a furnace, lengthening the metal material with a roughing mill and a finishing mill to form a long, thin metal material, and coiling the resulting material.
FIG. 13 shows an example of a commonly used hot-rolling line 100. A metal material (hereinafter, referred to as a “material to be rolled”) 8 having a thickness of 140 to 300 mm is heated to several hundreds to a thousand and several hundreds of degrees centigrade in a furnace 10 and is rolled with roughing mills 12 and a finishing mill 18, thereby forming a thin metal strip with a thickness of 0.8 to 25 mm.
In an exemplary structure shown in FIG. 13, two roughing mills 12 are arranged. Four roughing mills are commonly used. Furthermore, six roughing mills are used in some cases. The rolled material 8 is rolled with the roughing mills and then supplied to the finishing mill 18.
For example, the number of stands constituting the finishing mill 18 is seven in the exemplary structure shown in FIG. 13. In some cases, a finishing mill including six stands is used. The rolled material 8 having a temperature of several hundreds to a thousand and several hundreds of degrees centigrade is continuously rolled with the finishing mill 18 including the plural stands.
As shown in FIG. 13, a method in which a separate rolled material is rolled with the finishing mill 18 is referred to as batch rolling. In contrast, a method in which rolled materials are joined to each other and then rolled is referred to as endless rolling. It is more common to use the batch rolling.
The hot-rolling line 100 includes many (more than 100) table rolls (not shown) to transport the rolled material 8, except for the portions between the stands of the finishing mill 18.
Furthermore, the rolled material 8 has oxide layers (hereinafter, referred to as “scales”) on front and back surfaces thereof when discharged from the furnace 10. The rolled material 8 that is in a high-temperature state is exposed to air, thereby forming new scales on the front and back surfaces. Thus, descaling devices 16 for removing the scales by blowing high-pressure water having a pressure of about 10 to 30 MPa on the front and back surfaces are arranged on the entry sides of the stands of the roughing mills 12, and the scales are removed.
Work rolls 19 are cooled by cooling water (not shown) because these come into contact with the high-temperature rolled material. Backup rolls 20 are also cooled by cooling water.
In FIG. 13, reference numeral 14 denotes a crop shear. This removes crops of leading and trailing ends of the rolled material 8 (distorted portions of the leading and trailing ends of the rolled material 8) by cutting before the finishing mill to form the rolled material having a substantially rectangular planar shape that can be smoothly fed to the finishing mill 18.
Reference numeral 50 denotes a controller. Reference numeral 70 denotes a process computer. Reference numeral 90 denotes a business computer.
Meanwhile, in recent years, metal strips rolled in the hot-rolling line 100 as shown in FIG. 13 have been required to have higher quality. A representative example is a metal strip. Recently, trends toward the reduction in the weight of automobiles have placed higher demand on high-tension steel and require higher quality.
In general, high-tensile steel is used to indicate a steel sheet having a tensile strength of 400 MPa or more. In recent years, high-tensile steel sheets have been required to have not only high tensile strength but also high processability such that they are not cracked when subjected to press forming or burring. Furthermore, any portion of a metal strip has been required to have uniformity in quality such as tensile strength and high processability.
To produce high-tensile steel sheets, the chemical composition of steel is adjusted. Even if any chemical composition is used, hot-rolling technology and production conditions are important for the production of a high-quality metal strip. In particular, important points are a temperature of the metal strip immediately before coiling with a coiler 24 subsequent to finish rolling, and uniformization of the temperature of the metal strip in the longitudinal and width directions.
In the exemplary hot-rolling line 100 shown in FIG. 13, a temperature of the rolled material 8 measured with a thermometer 25, arranged on the entry side of the coiler, immediately before coiling is the most important for quality assurance. It is important to control a run-out table 23 and cooling-related equipment 26 arranged there. Furthermore, a temperature of the rolled material 8 measured with a thermometer 21, arranged on the delivery side of a finishing mill, immediately after rolling is also important.
To equalize the temperature immediately before coiling to the extent possible, the temperature immediately before coiling the rolled material 8 needs to be measured across the entire width of the rolled material 8. To control the run-out table 23 and the cooling-related equipment 26, preferably, a temperature of the rolled material 8 immediately after finish rolling needs to be measured across the entire width of the rolled material 8.
Hitherto, infrared radiation thermometers are typically used for the thermometer 21 arranged on the delivery side of the finishing mill and the thermometer 25 arranged on the entry side of the coiler. These thermometers are fixedly arranged above the laterally central portion of the rolled material 8 and have a field of view of at most 20 to 50 mm.
That is, a temperature of only the laterally central portion of the rolled material 8 is measured as a representative over the entire length. A temperature distribution in the width direction is not measured.
Even if the result of the temperature measurement of only the laterally central portion of the rolled material 8 over the entire length meets quality assurance standards, there is no guarantee that a temperature of the rolled material 8 in the width direction meets the quality assurance standards.
In batch rolling, an uneven portion is within several tens of meters from the leading end of the rolled material 8 because the flatness control effect of the finishing mill 18 is not exerted yet. Furthermore, an uneven portion is within a hundred and several tens of meters, corresponding to the distance between the final stand of the finishing mill 18 and the coiler 24, from the leading or trailing end of the rolled material 8 because no tension is applied thereto. The portions have distorted wave shape. For example, as shown in FIG. 14, pools of cooling water are locally present in several places in a leading end region of the rolled material 8. In such a case, these places are locally cooled; hence, it is difficult to achieve a uniform temperature distribution in the width direction.
Meanwhile, with respect to a phenomenon occurring between a surface of steel and cooling water, a rolled steel material having a temperature of 550° C. or higher is in a state of film boiling in which the entire surface of the rolled material 8 is covered with a continuous film of steam as shown in FIG. 15. At less than about 550° C., the film of steam disappears, and then the state is transferred to a state of nucleate boiling in which cooling water is in direct contact with the rolled material 8 as shown in FIG. 15b. In the case where the temperature of the overall rolled material 8 is further reduced, the state is totally transferred to the state of nucleate boiling.
In a state in which film boiling and nucleate boiling coexist, heat transfer is promoted in a portion in the state of nucleate boiling compared with a portion in the state of film boiling. Thus, a temperature of the portion in the state of nucleate boiling is lower than that of another portion surrounding the portion, in some cases.
A target temperature of a high-tensile steel sheet immediately before coiling is often 550° C. or lower to ensure the quality. The temperature corresponds to a temperature range in which the transition from film boiling to nucleate boiling occurs. Thus, film boiling and nucleate boiling coexist in a portion and another portion surrounding the portion of the rolled material 8; hence, there are a portion where the cooling rate is high and a portion where the cooling rate is low.
In the portions where the water pools are present as described above; low-temperature parts (black spots) are locally present in the rolled material 8, thereby further increasing the difference in temperature of the rolled material 8 immediately before coiling between the portions where the water pools are present and portions where no water pool is present. This leads to unevenness in the quality of the rolled material 8 as a whole, so that the quality of localized portions may fall outside an allowable range.
Efforts have been made to measure a temperature distribution of the rolled material 8 in the width direction. In recent years, the measurement has been becoming increasingly important.
In the past, to measure the temperature distribution of the rolled material 8 in the width direction, a separate thermometer configured to scan the rolled material 8 in the width direction has been arranged in addition to a thermometer fixedly arranged at a position corresponding to the laterally central portion of the rolled material 8. The temperature measurement was performed in such a manner that the rolled material 8 was scanned in the width direction while being transported, i.e., diagonal loci were plotted on the rolled material 8. As shown in FIG. 16 which is a view of a hot-rolling line when viewed from above, thus, localized low-temperature black spots were not scanned or detected, in some cases.
Japanese Unexamined Patent Application Publication No. 2005-279665 describes that a temperature distribution of a steel strip in the width direction after controlled cooling is discretely measured over the entire length of the steel strip. As shown in FIGS. 17a and 17b, the timing of the occurrence of the temperature deviation of the steel strip in the width or longitudinal direction coincides with timing of the initiation or termination of the operation of cooling-related equipment, such as cooling banks, nozzles, and headers, in some cases. It is described that part of low-temperature region of the rolled material 8 in the entire length and width as indicated by a black frame shown in FIG. 17a is determined to be an abnormal portion and that the cooling device is also determined to be abnormal. In Japanese Unexamined Patent Application Publication No. 2005-279665, it is speculated from FIGS. 17a and 17b that the temperature of the rolled material 8 in the width direction is discretely measured at a pitch of 200 mm.
Japanese Unexamined Patent Application Publication No. 2003-311326 described that, in the case of a plate-rolling line, a temperature distribution of a steel sheet is measured with a near-infrared camera and a scan-type radiation thermometer arranged on the downstream side (delivery side) of a hot leveler. The aim is to minimize the deformation of steel sheet due to the release of residual stress by determining a residual stress distribution and adjusting conditions of heat treatment, which is a post-production step.
A near-infrared camera includes, for example, a two-dimensional matrix of square pixels. Temperature data measured with the pixels is subjected to linear interpolation to determine a pseudo-continuous temperature distribution of an object. Longitudinal and lateral dimensions of one pixel each are smaller than 200 mm, which is an example of the pitch used for the discrete temperature measurement described in Japanese Unexamined Patent Application Publication No. 2005-279665. Thus, it is possible to measure a temperature distribution in a more continuous manner.
In Japanese Unexamined Patent Application Publication No. 2003-311326, although it is unclear that which portion of a rolled material is subjected to temperature measurement and how large the portion is, it is clear that the temperature over the entire width is not measured. For example, mention is made of a steel sheet having a width of 3,000 mm. A near-infrared camera capable of measuring the entire width of the wide steel sheet having a width of as large as 3,000 mm was not developed at the time Japanese Unexamined Patent Application Publication No. 2003-311326 was filed, and the near-infrared camera is not yet developed.
Japanese Unexamined Patent Application Publication No. 2000-313920 describes that, in the case of a hot-rolling line for a metal strip, a temperature of a surface temperature of a steel sheet during transport is measured on an upstream side (entry side) of cooling-related equipment. The aim is to reduce temperature deviation and achieve uniform quality to the extent possible by performing cooling control with cooling water when the minimum surface temperature is equal to or lower than a predetermined value and when the deviation of the surface temperature is equal to or lower than a predetermined value, or by performing cooling control with a cooling gas when the deviation of the surface temperature exceeds the predetermined value.
Japanese Unexamined Patent Application Publication No. 2000-313920 does not describe a near-infrared camera serving as means for measuring the surface temperature of the steel sheet is. Furthermore, it is also unclear that which portion of a rolled material is subjected to temperature measurement and how large the portion is.
The technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-279665 is based on the discrete temperature distribution measurement of a rolled material, the measurement being performed at a pitch of 200 mm. Like a traditional method in which a temperature of a rolled material is measured by scanning the rolled material in the width direction during transport, disadvantageously, a localized low-temperature portion, a black spot, is not detected, in some cases.
The technique disclosed in Japanese Unexamined Patent Application Publication No. 2003-311326 is targeted for a plate-rolling line. Furthermore, the entire width of a rolled material is not included in the measurement field of view. Thus, in the case where a localized low-temperature portion, a black spot, is present in a region out of the field of view, disadvantageously, the portion is not detected in the same way as above.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 2000-313920, from the viewpoint of the level of technology at the time the application was filed, there is a problem in which a shutter speed is not sufficiently fast. Furthermore, it is unlikely that the entire width of a rolled material is not included in the measurement field of view. Moreover, Japanese Unexamined Patent Application Publication No. 2000-313920 only describes the technique for controlling cooling with the cooling-related equipment by switching between water cooling and air cooling in a feedforward manner. A surface (two-dimensional) temperature distribution obtained as the result of the control is not measured. In addition, the measurement results are not recorded. Thus, disadvantageously, quality assurance for delivering a product to a customer cannot be provided.
It could therefore be helpful to provide a hot-rolling line, an electronic computer system configured to record a determination result of the quality of a hot-rolled metal strip, and an electronic computer system configured to control manufacture and quality histories and to control passing-step instructions, which provide proper quality assurance for delivering a product to a customer. In particular, these are characterized by assuredly detecting a localized low-temperature portion, a black spot.